Rocks of the Caldecott Tunnel

I mentioned a few posts back that I would be doing a field trip related to the new bore of the Caldecott Tunnel. Here’s how it went.

We (the members of the Northern California Geological Society) started with a talk at the Caltrans headquarters in Lafayette. They showed us historical photos and engineering diagrams, explained the New Austrian boring technique, and ascertained that we weren’t interested in a “photo opp” near the tunnel entrance. Apparently that last is a prime demand of many tour groups, but we didn’t care.

The tour leader unfurled a huge geologic map of the tunnel and talked a lot about all the rocks (click the image to see the 1000-pixel version). I studied it closely, because it’s the best we have for this part of town.

The blue stripes on the west side of the map are the Sobrante Formation, the green, purple and tan stripes are the Clarement Shale, and the buff stripe on the right is the Orinda Formation. These are stacked up in order of age, old on the west, and tilted nearly straight upright.

Next we wound our way around to the extreme top of Broadway, next to the tunnel’s west entrance. Here’s the view from there looking north.

The rocks in the immediate area are in the putative Sobrante Formation, but the Claremont formation takes over just to the east (right). Here’s the Sobrante exposed at that spot.

It’s pretty crummy rock, and the tunneling was quite slow there. Every couple of meters, the tunnelers drilled a fan of holes over the roof of the upcoming dig segment, each one reinforced and grouted. That reinforcement allowed the tunnel roof to stay up long enough to spray it with shotcrete, which in turn held up the roof until the real concrete tunnel lining could be emplaced. The tunnel’s final shaft is round, as strong as an eggshell in its resistance to the earth’s weight. (To see what I mean, try gripping a raw egg firmly in one hand, fingers on all sides, and attempt to crush it. You can’t do it.)

The unit here is mostly thin beds of chert and thinner interbeds of shale, but there are also lumpy beds in it—see one running down the center and another one near the right edge. Those are dolomite. It turns out that carbonate and silicate minerals dislike each other enough that they tend to segregate themselves: in chalks, you get flint lumps, and in cherts (as here) you get limerock lumps.

Our last stop was on Fish Ranch Road just east of the tunnel, where we looked at the Orinda Formation in the surrounding hillsides. The Caltrans people brought along a box of the fossils recovered from the fourth-bore project, so I got to lay my hands on this Miocene horse tooth . . .

. . . and this slab of fossil leaves.

The Caltrans public-information official was at obvious pains to say how happy the paleontology contractors were and how well the fossil collecting had gone. Obvious to me, anyway. I won’t forget the complaints I heard from the other party, but I will keep in mind that most stories have at least two sides, and that every fossil preserved is a victory over the universal decay of the world.

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